I'm looking to create a page that could perhaps become a Wikipedia article (or just remain in its current format). I'm sure that there are many more names that I could add. I also hope that it could potentially serve as a historical resource of value in its own right.
List of artists killed in the First World War.
Poets
Rupert Brooke
Julian Grenfell
Wilfred Owen
Isaac Rosenberg painter and poet
Charles Sorley
Arthur Graeme West poet and diarist
Edward Thomas
Guillaume Apollinaire d.1918 poet and critic a French poet, playwright, short story writer, novelist, and art critic of Polish descent.
Alan Seeger
Joyce Kilmer
John McCrae
Visual Artists
Henri Gaudier-Brzeska Fr. d.1915
Antonio Sant'Elia It. d.1916 architect
Umberto Boccioni It. painter and sculptor
August Macke Gr. d.1914
Franz Marc Gr. d.1916
Egon Schiele Aus. d.1918
Brian Hatton GB d.1916
Composers
George Butterworth GB d.1916
Ernest Farrar GB 1918
W.D.Browne GB 1915
Cecil Coles GB 1918
Frederick Kelly Anglo-Australian
Rudi Stephan Ger.
Alberic Magnard Fr. 1914
From the short biography chapter in "Minds At War" by David Roberts (a collection of WWI poetry). These are all poets; some of them only wrote one or two.
Phillip Bainbridge
Rupert Chawner Brooke
Julian Grenfell
William Noel Hodgson
Ewart Alan Mackintosh
Robert Palmer
Isaac Rosenberg
Alan Seeger
Charles Hamilton Sorley
Robert Ernest Vernède
Arthur Graeme West
I don't know if it interests you, but Walter Tull was a professional sportsman who was killed in action. He was the first black professional outfield soccer player, and he played mostly for Northampton Town. He was also the first black officer in the British Army to command white troops. He's the only one I know of, but there are bound to be many more professional sportsmen who died.
EDIT I should mention that I omitted the names you had, where I noticed that you already had them.
Robert Gregory, the only son of the Irish folklorist and dramatist Lady Gregory, was a painter who died over the Italian front in 1918. Her mother's friend, W. B. Yeats, memorialized Gregory in his poems "An Irish Airman foresees his Death and "In Memory of Major Robert Gregory".